1. Field of the Invention
The present invention relates to an exhaust system of an outboard motor and particularly to an exhaust system of an outboard motor in which a catalyst is properly disposed in an exhaust passage.
2. Related Art
An outboard motor discharges exhaust gas from an engine into water. To purify the exhaust gas, an outboard motor is provided with a catalyst in an exhaust passage of the engine. In such arrangement, if the catalyst comes in contact with seawater, for example, and flows back into the exhaust passage, performance of the catalyst is degraded. Therefore, it is preferable to dispose the catalyst in the exhaust passage in an engine cover located away from a water surface (sea surface).
Such arrangement of an outboard motor is, for example, disclosed in Japanese Patent Application Laid-open No. 2000-356123 (Patent Publication 1). In the outboard motor described in the Patent Publication 1, two catalysts are arranged side by side in the exhaust passage, and exhaust gas discharged from the engine is led to the exhaust passage above the engine and then flows through the two catalysts in the same direction to be purified.
In the outboard motor described above, however, the two catalysts are arranged side by side and the exhaust gas flows through the catalysts in the same direction, and therefore, according to increasing in capacity of the catalysts, a flow passage area of the exhaust passage increases and the exhaust gas flow is likely to stagnate. Moreover, the exhaust gas is led to the exhaust passage above the engine before it is led to the two catalysts, the exhaust passage in the engine cover increases in size.
As described above, the outboard motor of the Patent Publication 1 provides problems of increase in size of the exhaust passage and degradation of exhaust purifying performance.
Moreover, in the outboard motor described above, because the catalysts are arranged in side by side and the exhaust gas flows through the catalysts in the same direction, it is necessary for the exhaust gas to be distributed into two passages and led to the respective catalysts. In this case, if the exhaust gas is distributed unevenly, different amounts of the exhaust gas pass through the respective catalysts. Therefore, the catalyst through which excessive exhaust flows cannot sufficiently purify the exhaust gas and, hence, the purifying efficiency of the catalysts may be decreased.
Furthermore, in the outboard motor described of the Patent Publication 1, it is necessary to adjust gaps between the exhaust passage and the catalysts to make sure that the exhaust gas passes through the catalysts. In addition, in the outboard motor, the catalysts are supported by bolts, and when the bolts corrode, it may become difficult to attach or detach the catalysts.